1. Field of the Invention
This invention relates to the art of fertilizer and, more particularly, to a coated granular fertilizer in which the dissolution rates of water-soluble fertilizer components are controlled by coating or encapsulating a quick-acting granular fertilizer with a water-resistant film on the surfaces thereof.
2. Description of the Prior Art
In recent years, encapsulated or coated granular fertilizers have been in use wherein the granular fertilizer is encapsulated so that effective fertilizer components are controlled in dissolution rate.
We proposed a series of techniques for coated fertilizers using olefin polymers and copolymers (which may be hereinafter referred to simply as olefin polymer) as essential ingredients for a coating film. For instance, Japanese Patent Publication Nos. 50-99,858 and 60-3,040, assigned to the present assignee, disclose coating methods and coated fertilizers wherein olefin polymers with or without inorganic fillers are used as an encapsulating material.
For facilitating dissolution of the coated fertilizers, the Japanese Patent Publication Nos. 50-99,858 and 54-3,104 describe the use of nonionic surface active agents. Moreover, Japanese Patent Publication No. 60-37,074 describes the control of the dissolution rates by using a mixture of an ethylene-vinyl acetate copolymer and an olefin polymer at a suitable ratio.
According to these patent publications, in order to properly control the dissolution rates by covering a granular fertilizer with polymer compositions comprised primarily of olefin polymer and copolymer and an inorganic filler, it is necessary to balance the following requirements: (1) the ratio between an olefin polymer and an ethylene-vinyl acetate copolymer; (2) a content of surface active agent; and (3) a content of inorganic filler.
However, the control of the dissolution rates by varying the proportions or contents of the ingredients in these compositions places limitation on the maintenance of properties of the coating film. For instance, if the content of inorganic filler is increased, the dissolution rate increases with a lowering of film strength, thus presenting a problem of breakage during handling. Therefore, it is not convenient to increase the filler to a substantial extent. .
Although it is ordinarily preferable to control the dissolution rates by varying the ratio between the olefin polymer and the ethylene-vinyl acetate copolymer, this ratio is eventually determined by setting a dissolution rate of an effective fertilizer component. The attendant problem is that the physical and mechanical properties of the resultant film are also disadvantageously determined.
For instance, it is necessary to increase the ratio of the ethylene-vinyl acetate copolymer so as to increase the dissolution rate. The resultant film becomes soft. In coated fertilizers having different dissolution rates of the respective effective fertilizer components, it is not favorable from the standpoint of quality control that the coating films for the respective components have different physical properties. This may be overcome by controlling an amount of a surface active agent used. However, if surface active agents are used in large amounts in the olefin polymer film, there will arise the problem that the film is cracked owing to stress cracking with a loss of coating performance.
These problems may be solved when using in combination a soft polymer such as an ethylene-vinyl acetate copolymer, but if the surface active agent used is present around coated fertilizer granules in large amounts, the dissolution rate is facilitated. In commercial fertilizer products, the surface active agent is added in amounts within a range not presenting any problem on the dissolution rate. The control of the dissolution rate by incorporation of surface active agents in large amounts thus involves a problem on stability.